The surface state of an article often has an important effect on the performance of the article. For example, corrosion and oxidation damage resulting from exposure to an adverse environment are largely controlled by the character of the surface of the article. A protective layer may in some circumstances be applied to the surface of the article to improve its corrosion and oxidation resistance.
In another example, the fatigue resistance of an article, which is subjected to fatigue loading in service, is improved if the surface of the article is in a residual compressive stress state. The residual compressive stress state suppresses the initiation and propagation of fatigue cracks that would otherwise start at the surface and propagate into the interior of the material. Techniques for producing a compressive stress state at the surface are known.
However, such techniques for producing a residual compressive stress state at the surface may modify other properties of the surface, such as its smoothness, and consequently interfere with the ultimate performance of the article in its intended application. An airfoil whose surface is mechanically worked to produce a residual compressive state at the surface, thereby improving the airfoil's fatigue performance, may have its surface finish disrupted to such a degree that the aerodynamic performance of the airfoil is degraded. The known approaches for improving the surface finish in turn adversely affect the residual compressive stress state. Consequently, achieving both a desirable residual compressive stress state and the necessary high-quality surface finish has not been possible using existing techniques.
There is a need for an improved approach to the surface processing of airfoils and other shapes that achieves both the desired mechanical residual compressive stress state and the desired low-roughness surface finish. The present invention fulfills this need, and further provides related advantages.